[GHC] #9857: GHC 7.9 panics (simplifier ticks exhausted) on `half-0.2`

[GHC]

#9857: GHC 7.9 panics (simplifier ticks exhausted) on `half-0.2`
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              Reporter:  hvr         |            Owner:  simonpj
                  Type:  bug         |           Status:  new
              Priority:  high        |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.9
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Compile-    |       Blocked By:
  time crash                         |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Fixed by this (whose commit message contains a mis-typed ticket number)
 {{{
 commit fbb42b2ea42b6467135f26db47d9c296e7ad75a3
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Sat Dec 13 22:47:01 2014 +0000

     Pattern-synonym matcher and builder Ids must be *LocalIds*

     This easy-to-make mistake meant that pattern-synonym matcher and
     builder Ids weren't being treated as locally defined by the simpplier.
     That meant that we never looked up them up in the environment, got an
     out-of-date unfolding, which made the Simplifier fall into an infinite
     loop.  This was the cause of Trac #98587, but it was quite tricky to
     find!

     In a separate patch I'll make Lint check for locally-bound GlobalIds,
     since they are always an error.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9857#comment:6>
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